1. Field of the Invention
The present invention relates generally to heads-up displays and more particularly to heads-up displays employing a combiner with a thin dielectric film for providing high reflectance and high photopic transmittance.
2. Description of the Prior Art
In modern aircraft, both military and commercial, it is important that certain information be clearly and unobtrusively presented to the pilot while he is viewing the outside world. The heads-up display (HUD) has been developed for this purpose and provides relevant scale, alphanumerics, symbology, and other information displays superimposed on the pilot's forward field of view. With conventional HUD designs, as illustrated in an article by Leonard Ravich in the April 1986 issue of Laser Focus/Electro-Optics, the information display is generated on a high-brightness cathode ray tube. The information display is projected through a relay lens system to a mirror which reflects the display through a collimating lens onto a transparent combiner element which is located between the pilot and the aircraft windscreen. The combiner reflects the projected images to the pilot's eyes while affording him an unobstructed view of the outside world through the combiner and windscreen.
A typical HUD combiner is composed of a glass substrate upon which a holographic film is deposited for reflection of the projected image. This film comprises one or more layers of photoreactive gelatin which have been exposed by a holographic process in order to record a holographic fringe pattern and which have been subsequently developed by wet chemical processing as disclosed in Holographic HUDS De-mystified by J. H. Gard in the 1982 issue of IEEE National Aerospace and Electronics Conference Proceedings held in May 1982. The glass substrate and holographic film are then encased by a glass cover plate for environmental protection.
It is usually desirable to construct a hologram with fringes that are parallel to the surface of the gelatin. However, design constraints, such as optically recorded noise patterns, may prevent this construction. In these cases, the fringes intersect the surface and form a slant fringe pattern which produces extraneous diffraction images. The extraneous diffraction is termed "flare".
A common problem of holographic combiners as a result of flare is the diffraction of points of light, such as the beams from airport runway lights, when viewed through a holographic combiner. Diffraction produces multiple images of the light sources. Under certain conditions, particularly in night landings substantial portions of the pilot's field of view may be obscured by the flare from runway lights.
Another limitation with holographic combiners is the environmental instability of the gelatin layer. The holographic combiner must be covered by glass plates in an attempt to protect the gelatin film from high humidity conditions which would destroy it. The glass cover plates also serve to protect the gelatin from mechanical abrasive damage of which the gelatin film is extremely susceptible. Since the gelatin film is an organic, high temperature can chemically destroy it, so the film must be protected from high temperature exposure also. An additional problem encountered with holographic combiners is the complex wet chemical development which is required to fabricate the gelatin.
A further problem with such HUD combiner is the tendency of the layers of the film to delaminate both from the substrate and from other, adjacent layers therein. Such delamination is due to the differing coefficients of expansion between the various layers of the film such that as the combiner is subjected to varying thermal conditions, the layers expand or shrink at varying amounts resulting in delaminations between the layers. It would be desirable to develop a HUD combiner which did not exhibit flare or require special environmental protection from high humidity or high temperature conditions. Furthermore, it would be desirable for a HUD combiner not to require a gelatin film which necessitates complex wet chemical development processing and which is extremely sensitive to mechanical abrasion damage. Additionally, it would be desirable for a HUD combiner to not delaminate between the layers comprising the film.